To comply with the 2014 European and U.S. emission legislation (EU Stage IV and Tier 4 (final)) for non-road diesel engines, these diesel engines must be equipped with an exhaust gas cleaning system. Typical exhaust gas cleaning systems include, in addition to a diesel oxidation catalytic converter for oxidatively removing carbon monoxide and hydrocarbons, and, if necessary, a diesel particulate filter situated downstream therefrom, also a nitrogen removal unit. Units for selective catalytic reduction of nitrogen oxides with the aid of an SCR catalytic converter (SCR: Selective Catalytic Reduction), and a device for feeding ammonia or a compound decomposing to form ammonia in the exhaust gas stream to be cleaned, are typically used as the reducing agents for removing nitrogen oxides from diesel engine exhaust gases in utility and non-road vehicles. Preferred reducing agents include an aqueous urea solution or ammonium carbamate solution; particularly preferred is a urea solution. Such SCR units are typically situated downstream from an upstream diesel oxidation catalytic converter (DOC) and/or a diesel particulate filter (DPF).
To comply with the emission standards of EU Stage IV or Tier 4f, systems according to EP-B 1 054 722 or systems without diesel particulate filters (only DOC+SCR), for example, are used. In particular, in the last-named “open” systems, maximum SCR efficiencies must be achieved at all operating points even after extended operation, since the combustion process in engines, for the exhaust gas cleaning of which a system without diesel particulate filters is used, is adjusted in such a way that the lowest possible particulate emissions always occur. This causes significantly higher raw NOx emissions, so that nitrogen oxide conversion rates higher than 90% are required in the SCR unit over the entire time of operation of the system in order to reach the legal emission limit values.
Efficiencies of the SCR system are determined, in addition to the temperature and mass flow over the SCR catalyst, by the NO2/NOx ratio upstream from the SCR catalyst and by the quantity of reducing agent fed. The NO2/NOx ratio is adjusted via the upstream exhaust gas cleaning units DOC and/or DPF, preferably values of 0.2 to 0.7, and particularly preferably of 0.4 to 0.6 being achieved.
Feeding too little reducing agent, (for example, α=0.8. where α is the molar ratio NH3 to NOx in the exhaust gas to be cleaned upstream from the SCR catalytic converter) results in limiting the theoretically possible nitrogen oxide conversion corresponding to the availability of the reducing agent (i.e., for α=0.8, max. 80% nitrogen oxide conversion). By feeding an excess of reducing agent (α>1), the maximally thermodynamically possible nitrogen oxide conversions may be achieved over this SCR catalytic converter, which are determined only by the material properties of the catalytic converter under the given operating conditions (exhaust gas mass flow, temperature, NO2/NOx upstream from the SCR).
However, feeding an excess of reducing agent may result in ammonia breakthroughs through the SCR catalytic converter. Since ammonia is a poisonous and environmentally hazardous gas according to the EU hazardous substances designation, residual emissions must be avoided.
Systems according to the prior art typically regulate the reducing agent feed with the support of a model, i.e., the software stored in the engine control unit computes, on the basis of the NOx level in the raw emission and the previously experimentally ascertained efficiency of the SCR catalytic converter, the stoichiometrically required quantity of reducing agent at any conceivable operating point and controls the quantity of urea solution to be fed (pre-control quantity) accordingly. This pre-control is made difficult by the fact that, in particular, SCR catalytic converters, on the basis of zeoliths exchanged by transition metals, have a significant ammonia storage capacity. The quantity of ammonia stored in the catalytic converter depends on the operating temperature and the aging condition of the catalyst. Accordingly, depending on the operating point, part of the reducing agent quantity fed is used for filling up the ammonia accumulator in the catalytic converter. The accumulator may, in particular during dynamic operation, compensate for briefly occurring underfeeds by reducing the nitrogen oxides contained in the exhaust gas by using the ammonia desorbing from the accumulator. The ammonia accumulator must then be refilled by an overfeed of reducing agent.
This accumulator function of the catalytic converter makes optimum adaptation of the pre-control model difficult, since, due to the complexity of the chemical-physical processes in the SCR catalyst, they are very difficult to describe mathematically. Model-supported regulation of the reducing agent feed therefore has the disadvantage that, in particular during the transient operation of the engine, maximum efficiencies of the SCR catalytic converter cannot be ensured at all operating points without ammonia breakthroughs.
SCR systems in which ammonia breakthroughs downstream from the SCR catalytic converter are recognized with the aid of an ammonia sensor are known from the prior art.
Thus, WO 2010/062566 discloses the construction and mode of operation of an ammonia sensor.
DE 10 2006 051 790 discloses an exhaust gas treatment system for cleaning exhaust gases of an internal combustion engine, including in the direction of flow of the exhaust gas, in this order, a first oxidation catalytic converter, a device for introducing fuel into the exhaust gas tract, a second oxidation catalytic converter, a diesel particulate filter, a device for injecting a reducing agent that is effective in reducing nitrogen oxides, an SCR catalytic converter, and, in some cases, an ammonia anti-slip catalyst having an oxidation catalytic effect. Downstream from the SCR catalytic converter, an ammonia sensor may be provided for improving the regulation of the reducing agent feed or for diagnostic purposes.
EP-A-2 317 091 discloses an exhaust gas cleaning system including, in the direction of flow of the exhaust gas, in this order, an oxidation catalytic converter, an exhaust pipe having a feeding device for urea solution and an SCR catalytic converter. A temperature sensor is integrated into the SCR catalytic converter. Downstream from the SCR catalytic converter, an ammonia sensor is provided for detecting the ammonia concentration in the exhaust gas downstream from the SCR catalytic converter. An ammonia oxidation catalytic converter may be situated downstream from the ammonia sensor. In the system disclosed in EP-A-2 317 091, a quantity of urea solution to be fed (pre-control quantity) is determined and fed as a function of the rotational speed and torque of the engine. At the same time, the ammonia storage capacity of the SCR catalytic converter is computed from the time delay between the start of feed and the beginning ammonia slip. If an ammonia slip is shown by the ammonia sensor downstream from the SCR catalytic converter, the quantity of urea solution to be actually fed is reduced with respect to the pre-control quantity. If the computation of the ammonia storage capacity of the SCR catalytic converter provides a value that is less than a reference value stored in the control software, the quantity of urea solution to be actually fed is increased with respect to the pre-control quantity.
EP-A-2 317 090 discloses a method for operating an SCR system, in which the reducing agent feed is preventively reduced when ammonia breakthroughs through the SCR catalytic converter are to be expected due to the operating conditions. Such changes in the operating conditions include, in particular, changes in the exhaust gas mass flow and/or a rise in the exhaust gas temperature. EP-A-2 317 090 also discloses a method for detecting an ammonia slip risk with the aid of an ammonia sensor situated between two SCR catalytic converters. If a predefined ammonia slip is exceeded downstream from the first upstream SCR catalytic converter, the reducing agent feed is discontinued.
DE 10 2008 043 141 discloses an exhaust gas cleaning system for a diesel engine, including, in the direction of flow of the exhaust gas, in this order, a diesel oxidation catalytic converter, a device for feeding ammonia into the exhaust gas tract, an SCR catalytic converter, an NOx sensor for detecting nitrogen oxides in the exhaust gas, an ammonia oxidation catalytic converter, a device for feeding water into the exhaust gas tract, and an ammonia sensor. If an ammonia concentration exceeding a predefined value is detected in the exhaust gas by the ammonia sensor downstream from the ammonia oxidation catalytic converter, water is fed into the exhaust gas tract downstream from the ammonia oxidation catalytic converter in order to “capture” the ammonia present in the exhaust pipe and thus prevent the escape of ammonia into the atmosphere.
US 2009/0272099 and US 2010/0242440 disclose exhaust gas treatment systems including, in the direction of flow of the exhaust gas, in this order, an oxidation catalytic converter, a diesel particulate filter, a device for feeding a reducing agent such as ammonia or a urea solution, an SCR catalytic converter, and an ammonia oxidation catalytic converter. Ammonia sensors may be situated downstream from the ammonia oxidation catalytic converter, upstream from the SCR, and/or upstream from the ammonia oxidation catalytic converter. They are supplemented by NOx sensors for detecting the nitrogen oxide level in the exhaust gas upstream from the diesel oxidation catalytic converter, downstream from the SCR and/or downstream from the ammonia oxidation catalytic converter. With the aid of these sensor signals, the actual reducing agent feed rates are adjusted in such a way that suboptimum reducing agent feed rates occurring due to errors or inaccuracies in the pre-control model (e.g., modeling errors, deviations in the actual efficiencies due to catalyst aging or sensor aging, deviations in the reducing agent concentration, injection delays) are corrected.
WO 2011/139971 discloses a method for operating an SCR system which has two SCR catalytic converters situated in series in the direction of flow of the exhaust gas, and an ammonia sensor between the two SCR catalytic converters, as well as an NOx sensor downstream from the second, downstream SCR catalytic converter. The method is characterized by the fact that the predefined value for the ammonia concentration in the exhaust gas prevailing between the two SCR catalytic converters, which is determined with the aid of the ammonia sensor, is modified or adjusted as a function of the NOx concentration determined with the aid of the NOx sensor in the exhaust gas downstream from the second SCR catalytic converter.